This invention relates to oxidatively stabilized adhesion promoters for hot-melt adhesives which comprise stabilized poly(vinylalkylethers) of improved oxidative stability containing an initial level of hydroperoxides up to about 1000 parts per million before exposure to elevated temperatures. The oxidatively stabilized poly(vinylalkylethers) have a carbonyl index of less than about 2.9 after exposure to high temperatures for prolonged periods. The oxidatively stabilized polymers have a number average molecular weight of from about 5,000 to about 100,000 and contain from about 0.01 weight percent to about 2.00 weight percent of polymerized 1,2-dihydro-2,2,4-trimethylquinoline as an oxidative stabilizer and antioxidant. Secondary antioxidants such as organic compounds of sulfur and trivalent phosphorus are optionally present.
This invention also relates to improved hot-melt adhesive compositions effective in binding polyolefins to themselves and to other materials which are based upon an oxidatively-stabilized poly(vinylalkylether) and, more particularly, to hot-melt adhesive compositions which strongly bind polyolefins, in particular polyethylene, polypropylene, and ethylene-propylene copolymers, to themselves and to other substrates including Kraft paper, burlap, glass and metals, and which are formulated from an oxidatively-stabilized poly(vinylmethylether) (PVME) a thermoplastic resin, and, optionally, a tackifier resin. The PVME is stabilized against color degradation under prolonged heat at elevated temperatures.
Polyvinyl alkyl ethers are known to be subject to oxygen, heat, and light and to suffer depolymerization or breakdown in molecular weight to lower molecular weight polymers and to the alcohols from which the monomers are derived. The alcohols can further oxidize to aldehydes, ketones and acids. A number of stabilizers and antioxidants have been proposed to overcome the influence of oxygen, heat and light which cause chain cleavage, cross-linking and reactions with oxygen. These aging processes affect the polymer's adhesive properties and can cause degradation of the polymer to form a gel-like substance and/or a skin formation.
It has been taught, U.S. Pat. No. 2,985,617, that polymers of 2-3 carbon atoms such as polymerized vinyl ethers prepared by use of a Ziegler catalyst, such as the material formed from a trialkylaluminum with titanium tetrachloride, can be stabilized against the effect of thermal processing by a small amount of a polyvinyl chloride heat stabilizing agent selected from the group consisting of alkaline earth metal salts, epoxidized organic oils and esters, epoxy resins, organo-tin compounds, organo-lead compounds, organic phosphites, and substituted ureas. Weak bases are taught as being useful in stabilizing polymers of 2-3 carbon atoms against the effects of thermal processing.
U.S. Pat. No. 2,521,950 teaches that incorporation of small amounts of p-hydroxy-N-phenylmorpholine in polymerized vinyl alkyl ethers can retard depolymerization of these polymers by heat. However, the use of this compound as a stabilizer suffers from the disadvantage of discoloration of the polymer. The addition of an amount of an alkali so that the polymer has a pH value of at least 9 as measured by a glass electrode is taught as a means of improving the color of the polymer containing the heat stabilizing compound.
Polymerized 1,2-dihydro-2,2,4-trimethylquinoline is known in the art as a stabilizer of polyesters for protection against degradation due to heat and oxygen but discoloration of the stabilized polyester occurs. For example, U.S. Pat. No. 3,620,824 teaches use of 2,2,4-trimethyldihydroquinoline to stabilize polyethylene terephthalate modified with polyethers such as methoxy polyethylene glycol. The presence of a alkylated hindered bisphenol is required to prevent the discoloration of the stabilized polyester caused by the use of 1,2-dihydro-2,2,4-trimethylquinoline.
Hot-melt adhesives containing polyvinylmethylether are known in the art. U.S. Pat. No. 2,970,974 teaches a hot-melt adhesive compatible composition consisting essentially of up to 75 wt. percent poly(vinylmethylether) and a modifying agent selected from the group consisting of rosin and ester derivatives thereof. U.S. Pat. No. 5,080,978 teaches a hot-melt adhesive comprising a poly(vinylalkylether), preferably a poly(vinylmethylether), and a thermoplastic resin selected from the group consisting of C.sub.2 to C.sub.6 polyolefins and copolymers. Other examples of polyolefins utilized in hot-melt adhesives are ethylene-vinyl acetate copolymers and styrene-isoprene block-copolymers.
Because of their attributes, it is expected that hot-melt adhesives will tend to be more the adhesive of choice in high speed packaging operations. Increase in packaging line speeds to 400 or more cartons per minute has made instant setting adhesives essential. Hot melt adhesives with improved characteristics are therefore of continuing interest.
Hot melt adhesives are typically used or applied at temperatures of from about 170.degree. C. to about 210.degree. C. They are therefore subject to decomposition as evidenced by the development of discoloration, skinning, charring or change in viscosity. The purpose of an antioxidant is to maintain the original properties of the hot-melt adhesive, so as to be thermally stable and permanent. The loss of antioxidant activity from hot-melt adhesives therefore can reduce the utility of the hot-melt in a typical application.
Hot-melt adhesives can be prepared in granular form, lump form or as a rope-like material. They can be applied to the substrate by several different methods. Frequently, the hot-melt adhesive is made fluid by heating in a large feed-stock vessel. The hot-melt adhesive is then applied to the substrate with the aid of rolls or pumps. Pot life of the hot-melt adhesive, i.e., the hot-melt adhesive remaining fluid at the elevated temperature for long periods of time without depolymerizing or discoloring or developing a skin, becomes of critical importance in maintaining packaging line speeds and product quality.
Polyethylene, polypropylene, and ethylene-propylene copolymers are widely used in many applications which require adhering these plastics to themselves as well as to other materials. However, adhesives do not form strong bonds to untreated polyolefin surfaces. The difficulties encountered in developing adequate adhesion to polyethylene and polypropylene can be attributed in large part to the non-porous and non-polar characteristics of these materials. In addition, the presence of surface materials such as mold release agents, slip agents, antioxidants, polymerization impurities, and low-molecular-weight surface polymers further deteriorates surface conditions for adhesion. Various surface preparation procedures, including etching with sodium dichromate-sulfuric acid solution, treating with flame, corona discharge, electron beam, laser light, ultraviolet radiation, hot chlorinated solvent, and applying chemically reactive primers have been developed to afford a polyolefin surface that is more receptive to conventional adhesives. However, such pretreatment adds significant cost to a finished assembly and can reduce the economic incentive to use polyethylene or polypropylene in place of more expensive plastics. Further, some surface treatment procedures, e.g., corona discharge, are only applicable to thin, flat surfaces and cannot be used in many existing applications.
Poly(vinylalkylethers) have been widely used in adhesive formulations, the specific alkyl group of methyl, ethyl, isopropyl, n-butyl, isobutyl, t-butyl, stearyl, benzyl and trimethyl determining to a large extent the hot-melt adhesive application. Of the alkyl groups, the methyl ether compound offers a larger potential number of applications as a component of a hot-melt adhesive formulation. Improvement of poly(vinylmethylether)-based hot-melt adhesives by use of an oxidatively-stabilized poly(vinylmethylether) to reduce tendency to oxidative degradation and gel and skin formation will therefore be of wide utility in the art because of the large number of possible applications. Typical applications for poly(vinylmethylether) will be in bonding polyethylene, polypropylene and ethylene-propylene copolymers.
A hot-melt adhesive with improved oxidative resistant properties which strongly bonds untreated polyethylene, polypropylene, and ethylene-propylene copolymers would represent a significant achievement and present opportunities to not only market a new family of adhesives, but also open many new markets for such polyolefins in the areas of automobiles, appliances, fabrics, etc. Additionally, hot-melt adhesives with improved oxidative-resistant properties such as with use of the invented composition would find extensive use with other substrates.
Poly(vinylalkylethers) and particularly poly(vinylmethylether) have long been used in hot-melt adhesive formulations with many different antioxidants. One such antioxidant is the 3.5-di-tert-butyl-4-hydroxyhydrocinnamic acid triester of 1,3,5-tris-(2-hydroxyethyl)-a-triazine-2,4,6-(1H.3H.5H) trione with a molecular weight of 1042 commercially available under the trademark "Vanox SKT." Other antioxidants that have been used include, for example, tris(di-t-butyl-p-hydroxybenzyl)trimethylbenzene (Ionox 330), alkylated bisphenol (Naugawhite), zinc dibutyl dithiocarbamate (Butyl Zimate), and 4,4'-methylene bis(2,6-di-tert-butylphenol) (Ethyl 702), tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate) methane] (Irganox 1010), lauryl stearyl thiodipropionate (Plastanox 1212), and dilauryl 3,3'-thiodipropionate (Plastanox LTDP), 2,6-di-tert-butyl-p-cresol (BHT) and the like.
However, such antioxidants do not provide sufficient protection against oxidative degradation to a poly(vinylmethylether) hot-melt formulation at high temperatures over an extended period of time as measured by the carbonyl index of aged samples as determined by infrared spectra.
Now it has been found that an improved poly(vinylmethylether) hot-melt formulation with improved protection against oxidative degradation at high temperatures over an extended period of time as measured by the carbonyl index of poly(vinylmethylether) formulations can be obtained by incorporating a stabilizer, polymerized 1,2-dihydro-2,2,4-trimethylquinoline into a poly(vinylmethylether) before the poly(vinylmethylether) has a hydroperoxide content greater than about 1000 parts per million (ppm), preferably less than about 500 ppm, more preferably less than about 300 ppm. An initial presence of greater than 1000 ppm of hydroperoxides reduces the oxidative stabilization of the poly(vinylmethylether) to an unacceptable level when the poly(vinylmethylether) is exposed to an elevated temperature for a prolonged period.
It is therefore an object of this invention to provide a new and useful adhesion promoter composition comprising PVME and an antioxidant, the composition initially containing up to about 1000 ppm of hydroperoxides measured at room temperature, and wherein the carbonyl index of the adhesive composition after exposure to 350.degree. F. for a period of up to about 104 hours is no more than about 2.9.
It is still a further object of this invention to provide a composition of PVME with excellent characteristics as to color and heat stability, with improved resistance to discoloration, skinning and charring.
It is still further an object of this invention to provide a new and useful adhesion promoter composition comprising PVME characterized by outstanding oxidatively stabilized properties. The oxidatively stabilized composition comprising PVME is employed in hot-melt formulations to obtain improved characteristics as to the lessened development of discoloration, skinning, charring, or change in viscosity upon prolonged application of heat.